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DC ELECTRICAL CIRCUITS

INDUCTANCE

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DC ELECTRICAL CIRCUITS

When current travels down a conductor it creates a magnetic field around the conductor, this magnetic field induces an EMF onto the wire and will also induce a voltage onto other conductors running in parallel with the current carrying conductor.

CURRENT FLOW

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DC ELECTRICAL CIRCUITS

Inductance is the ability of a device or circuit to oppose a change in current flow.

Induction is the action of inducing an EMF when there is a change in current.

The unit of measure for inductance is the henry (H) and the symbol for inductance is (L).

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DC ELECTRICAL CIRCUITS

Types of inductors;

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DC ELECTRICAL CIRCUITS

As you can see in the picture below an inductor can be nothing more than a coil of wire.

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DC ELECTRICAL CIRCUITS

The larger the conductor and the more coils it has creates a much larger inductive field, by wrapping the coil around iron the inductance increases greatly

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DC ELECTRICAL CIRCUITS

The symbols for inductors is pictured below.

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DC ELECTRICAL CIRCUITS

Most DC circuits reach the steady state condition within a fraction of a second after power is applied.

In the steady state condition the current has reached is calculated value per ohms law.

Because of other circuit characteristics the current does not reach steady state value instantaneously, there is a transient time in which the current builds up to its steady state value.

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DC ELECTRICAL CIRCUITS

During the time transient, when the current is changing from zero to its specified value, self induction occurs.

Self induction is the action of inducing an EMF into a conductor when there is a change of current in the conductor.

The transient time depends on the value of the inductance and the value of any resistance

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DC ELECTRICAL CIRCUITS

To determine the time constant of an inductor the formula is; T = L/R

In a circuit that consist of a .5mH inductor and a 1KΩ resistor the time constant is;

T = .005H/1000Ω or .000005uS

This means it will take 5uS for the current through the inductor to reach 63.2%

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DC ELECTRICAL CIRCUITS

If the maximum current through the circuit is 100A, the current after one time constant is;

100A x .632 = 63.2A

The current must increase another 36.8 amps before it reaches it’s maximum value.

During the second time constant the current increases by another 63.2% of the remaining 36.8A.

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DC ELECTRICAL CIRCUITS

The calculation for the second time constant is;

36.8A x .632 = 23.25A

Adding this to the first time constant;

23.25 + 63.2 = 86.45AAfter five times constants the current through the inductor is considered to be at its maximum value.

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DC ELECTRICAL CIRCUITS

Inductors in series;Lt = L1 + L2 + L3 + Ln

Inductors in parallel;